Why does an object with a glossy shader render in black?
What you are missing is proper lighting and an environment to be reflected on the object.
Since this question gets asked a lot I'll try to address some common misunderstandings on dealing with glossy objects in general, this shoudl be helpful not only black shiny objects, but metallic or glass ones
A glossy surface is like a mirror.
When you stand in front of a mirror what do you see?
Yourself and whatever is behind you...
Do you see the surface of the mirror?
Hardly, unless is dirty, fogged or scratched.
If you shine a flashlight into the mirror what happens? Does the mirror get brighter? Or is the light reflected back at you? What should you pay attention to, lighting the mirror or lighting the objects that are being reflected in the mirror?...
Problem #1
Glossy objects render black, glossy material is not working &quot;as expected&quot;:

A perfectly glossy object, with no roughness, will reflect all the light perfectly &quot;as expected&quot; If the scene has no environment or other objects, glossy surfaces will reflect... nothing: hence the blackness...
Problem #2
The glossy object is dark or I just see small bright dots
If my glossy objects are dark I can just add lights or make the light in the scene brighter and then my objects will not be dark.
Yes and No.
Adding more lights or making them brighter will not work as well, the tiny spots where the lights are reflected on the object will get brighter but the shape of the object will not be revealed.
Light rays hitting on a glossy surface or mirror like surfaces get reflected from the surface in an equal amount and a the symmetrical angle to the one of incoming light. – in comparison with diffuse reflection – in which the light gets scattered all along the surface of the object in different amounts on other directions, depending on roughness.
As an example:

Image by GianniG46
Light on a diffuse shader gets scattered all over the surface in many directions and reveals the complete shape of the object. All surfaces will reflect some light one way or another.

Light on a glossy shader bounces straight back or out of view, and will only visible in a very small part of the surface of the object: in this example is only a tiny little white spot.

So what that means in plain English is that that the lights, no matter how bright or how many they are in the scene, will only be visible at a certain angle. If they happen to be at the angle in which the light rays get reflected straight into the camera lens they will be visible as bright spots, but they are not really &quot;lighting&quot; the object.
Here's an example with a ridiculous amount of lamps, the more lamps the harder it is to understand the shiny objects.

Problem #3
If I just set the environment to be something else other than black things look odd and flat
Let's set the environment to be gray for example.

Yes! We're finally getting seeing the objects and reflections!... But wait.. it look very strange and flat indeed and I don't want a cartoon look...
Since the environment is irradiating light evenly in every direction it can be be hard to tell the object apart from the background. All surfaces are getting equal amounts of light she shapes are not clear.
In the image above there is some color on the glossy shader... but for other objects with no color, this kind of homogeneous lighting, the object could appear totally camouflaged: where the reflections are all essentially the same color and as bright as the background. To understand shapes we need a variety of values that will help our brain know if the object is straight of curved or if it has any depth.


OK I get it, but how do I see my glossy object properly?
To reveal the surface of a glossy object you need to reflect something on it.
In this case, a few large planes (or area lights) with an emission shader strategically placed at an angle to maximize reflections, will make our object finally look like something!

Start by placing a single very large plane with an emitter in the scene and see how is being reflected on the surface. Move it around the scene, and notice how the placement helps you reveal or not the shape and qualities of your object. Add additional emitters or just planes with a white surface. Find the correct placement for them. Having gaps in between the emitters/reflectors will help you reveal the contours and subtle shapes of the object.
But don't limit yourself to planes or lights, be creative. The challenge is finding something interesting to reflect on the surface of your objects!

Depending on what you are after, using an HDR image as the environment and light source might give you more natural reflections.

Look online for links on &quot;how to photograph reflective objects&quot;,  The same techniques apply for 3D rendering.
You will realize that they all do the same: they are not just photographing an object, but mostly focusing on what is being reflected on the surface of the object.
This link might help: http://project52pros.com/2014pros/assignment-thirty-five-photographing-a-very-shiny-object/